This invention relates to positional determination systems and devices such as digitizers and, more particularly, to a method and associated apparatus for dynamically setting the scanning rate of positional sensing elements of the device in direct relation to the speed of movement of a cursor over a sensing surface of the device so that the faster the cursor is moving, the faster the scanning rate and the shorter the time each sensing element is sampled.
Electro-magnetic digitizers are well known in the art. As depicted in FIG. 1, a typical electro-magnetic digitizer 10 comprises a tablet 12 over which a cursor 14 is moved to input positional information to a computer connected to the tablet 12. The tablet 12 contains a grid of wires used to sense the position of the cursor 14. There are a plurality of equally-spaced X-wires 16 and a plurality of equally-spaced Y-wires 18 perpendicular to the X-wires 16. Either the wires 16, 18 can be powered and the electro-magnetic signal from the wires 16, 18 detected in the cursor 14 or the cursor 14 can be powered and the electro-magnetic signal from the cursor 14 detected in the wires 16, 18. A powered cursor 14 is preferred for so-called "cordless" digitizers in which the cursor 14 has no physical connection to the tablet 12.
Since the wires 16, 18 are spaced and discrete, it is impossible to develop a continuous signal indicating the position of the cursor 14 in the coordinate system defined by the wires 16, 18. The wires 16, 18 must be scanned sequentially to determine the signal being developed in each as a result of the electro-magnetic signal from the cursor 14. Then, the position of the cursor 14 can be determined mathematically from the data using well-known techniques. Typically, the approach is as depicted in FIG. 2. First, the electronic associated with the tablet 12 scans (i.e. sequentially samples) the signals on the wires 16 of the X grid and then the wires 18 of the Y grid are scanned. This process repeats continually to develop the necessary signals for position determination of the cursor 14.
Within each grid sample period 20, the wires 16 or 18 of a particular grid are sampled sequentially as depicted in FIG. 3. Actually, each sample 22 of a wire 16, 18 is an integration process of the signal being induced into the wire by the electro-magnetic signal from the cursor 14. The integration period for each wire 16, 18 determines the length of each grid sample period 20 and the length of each grid sample period 20 determines the overall scanning rate of the tablet.
Early digitizers operating in the above-described manner had a fixed integration period for each wire 16, 18 as determined at the time of manufacture. More recently, digitizers operating in the above-described manner have been provided with a statically adjustable integration period for each wire 16, 18 as determined from operator inputs at the time of and prior to digitizing with the digitizer as depicted in FIG. 4. In this prior art approach, the logic of the digitizer first reads the inputs from the operator. It next sets the integration period for the wires 16, 18 as a pre-established function of the operator inputs. Digitizing is then accomplished using this pre-established integration period for the wires 16, 18 for all functions of the digitizer.
The problem is that a digitizer is a dynamic device which respond differently under different dynamic conditions. If the cursor 14 is unmoving at position X1, Y1 in FIG. 1 there is one response. If the cursor 14 is moving position X1, Y1 to position X2, Y2 slowly, there is a second response. If the cursor 14 is moving position X1, Y1 to position X2, Y2 rapidly, there is yet a third response. As the integration rate is reduced (i.e. the wires 16, 18 are scanned at a faster rate), the noise (resulting in data jitter) of the system gets worse. This is most evident when the cursor 14 is not moving. Thus, to reduce jitter when the cursor 14 is not moving or moving very slowly, one should employ a slow scanning rate. On the other hand, if the cursor 14 is moved rapidly employing a slow scanning rate, there can be considerable positional error in the signal. If one pictures the cursor 14 being moved in a curve at high speed at an extremely slow scanning rate (approaching the ridiculous for demonstration purposes), it will be realized that the cursor 14 can be "seen" at the start of the curve and not "seen" again until the end of the curve. Thus, the positional data developed will indicate that the cursor 14 jumped from the starting position to the ending position along an assumed straight line. In the prior art system of FIG. 4, therefore, the operator (and the tablet logic) are placed on the horns of a dilemma as to what integration rate to employ. Typically, a middle point is chosen such that if the cursor 14 is moved at medium speed the developed signal will be acceptable. While at rest, the cursor 14 may have signal jitter and during rapid movement, the cursor 14 may suffer from positional inaccuracies.
Other positional devices which scan sensing elements in relationship to a cursor moved over a sensing surface encounter a similar problem. It is particularly important to solve the problem in the pen-based computer systems presently being developed where the digitizing surface is associated with a relatively small display panel and the cursor is pen-shaped. In such systems, faithful reproduction of the pen movement in the data supplied to the computer is particularly important.
Wherefore, it is an object of the present invention to provide a method of operating electro-magnetic scanning digitizers to prevent signal jitter at slow cursor speeds and during cursor non-movement. non-movement.
It is another object of the present invention to provide a method of operating electro-magnetic scanning digitizers to prevent signal error at high cursor speeds and during cursor non-movement.
It is still another object of the present invention to provide a method of operating electro-magnetic scanning digitizers in which the scanning speed of the grid wires is dynamically adjustable.
It is yet another object of the present invention to provide a method of operating positional devices which scan sensing elements in relationship to a cursor moved over a sensing surface to prevent signal error at high cursor speeds and during cursor non-movement.
It is a further object of the present invention to provide a method of operating pen-driven computing devices having a pen-shaped cursor which is moved over a sensing surface to assure highly accurate positional reproduction and prevent signal error at high cursor speeds and during cursor non-movement.
Other objects and benefits of the invention will become apparent from the detailed description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.